There is an increased commercial need for the measurement of gases that are present in trace amounts, typically in concentrations of 100 parts-per-billion and less. Ion mobility spectrometry (IMS, previously known as plasma chromatography) is one particularly useful technique for the determination of analyte gases in trace amounts. Another particularly useful technique for detecting trace amounts of gases is metastable ionization detection (MID).
In IMS devices, such as that described in U.S. Pat. No. 5,457,316, the trace chemicals to be detected are ionized and then separated from each other due to their differing drift velocities in an electric field. The time differentials of their arrivals to a collector are then registered. Hence the name "ion mobility".
In metastable ionization detection (MID), a gas sample mixture is directed through a separator, such as a gas chromatograph column, wherein the trace gas(es) are delayed by varying time periods. The time-delayed gases are then introduced sequentially into a chamber wherein metastable atoms of a rare gas (such as helium, argon, neon, etc.) are generated which interact with and ionize the trace gases. The ionized trace gases are then detected by a collector and are positively identified by the amount of time delay.
While the IMS and the MID (with a gas chromatograph) devices can effectively identify trace amounts of certain gases, they typically require a radiation source, such as a nickel-63 beta-radiation source for ionization. Such an ionization source is impractical for some commercial and aerospace applications. Also, prior IMS and MID designs can require more supplies and power than is available for such applications.
Since the MID was first described in 1958 by J. E. Lovelock, many investigators have experimented with different electrode and housing configurations. One such configuration, which has a reported cavity volume of 80 microliters and is a current candidate for a space-qualified MID, is the miniature triaxial MID described in The Triaxial Metastable Ionization Detector, Anal. Chem. 56, 860-862, F. H. Woeller, D. K. Kojiro and G. C. Carle (1984). Work with this and other prior designs has shown that a reduction in detector volume often correlates to an increase in sensitivity, depending on certain factors such as the mechanism of detection, the quantity of gas available to analyze, and the noise or instability characteristic of the device.
The drive to develop ever smaller and more sensitive detectors is fueled by the availability of miniaturized gas chromatographs, as well as weight considerations and the limited availability of consumable gas supplies and other utilities in certain applications (such as space exploration). An innovation of this invention is the use of a novel structure for a Micro-Volume Metastable Ionization Detector (MVID) which achieves a goal of an active volume (not cavity volume) of less than one microliter (10.sup.-3 ml). The MVID is also adaptable to microbore and microchannel gas chromatographs, and because of the small internal volume of this detector, low gas flows (in the range of 1 ml/min) are permitted.
Generally, prior art MIDs have two functions--the generation of metastable atoms and the detection of the sample ions caused by the interaction of the neutral metastable atoms with the sample gases. The ions produced from interaction with metastable atoms generate a signal when detected by the collector, such as an electrometer. A detailed description of the initial ion generation processes in the type of electrode structure in the MVID can be found in J. D. Cobine, Gaseous Conductors, (Dover Publications: NY 1958), p 258, the disclosure of which is herein incorporated by reference.
In conventional metastable ionization detectors, there is usually only one electrical circuit. This one circuit measures the sum of all ion processes including both the radiation ionization (e.g. from tritium or nickel-63) and the ion multiplication from the electron flow across the gap. In addition, the detection current is mixed in with this current and, since the threshold is only about 1/100th of the standing current, a large part of the electrometer current must be subtracted in order to measure the signal current.
In the MVID, the two functions of generation and detection are decoupled. Therefore the currents and voltages, and even the gap spacings, can be different. Thus the two functions can be optimized independently of one another.
Specifically, one embodiment of an MVID made in accordance with the present invention comprises an insulating body with a small (40-60 microliter cavity volume) center core into which three electrode tubes extend. An outer electrode tube is disposed around and aligned concentrically with an inner electrode tube. A collector electrode, such as an electrometer electrode tube, is aligned coaxially with the inner electrode tube in an end-to-end fashion within the center core such that the collector electrode tube has an end face spaced a short distance from an end face of the inner electrode tube within the center core. The two end faces partially define a small active volume (1 microliter) of the center core. The inner electrode tube is connected to a sample gas inlet and the outer electrode tube is connected to a corona gas inlet. The collector electrode is connected to a gas outlet. A disc extends radially inwardly from the outer electrode tube forming a narrow annular corona gap between the disc and the inner electrode. Separate power supplies are connected to the inner and outer electrodes to create a corona discharge across the annular corona gap. A collector, such as an electrometer, and read out instruments are connected to the collector electrode to detect ions generated in the MVID as they exit through the electrometer tube.
In the operation of the MVID, a mixture of sample analyte gas(es) and a carrier gas is introduced into the center core through the inner electrode. A corona gas, which is a substantially pure form of the carrier gas, is introduced into the center core through the outer electrode tube. As the corona gas passes through the corona gap, metastable forms of the corona gas are created which flow into the center core in a concentric manner with the sample gas. In the small active volume between the inner electrode and the collector electrode, the metastable species interact with and ionize the analyte sample gas(es). The analyte ions formed are then detected as the gas mixture exits the MVID through the coaxially-aligned collector electrode.
The MVID of the present invention has an active volume of under 1 microliter and a cavity volume of only 40-60 microliters. In addition, the dimensions and spacing of the electrode tubes are in the millimeter range. These small volumes allow the MVID to accommodate low gas flows of about 1 ml/min thereby resulting in low consumption of gas samples. Also, since the MVID uses an electronic corona discharge to ionize the gas samples, no radiation source is required. In addition, the signal current and ion generation current in the MVID are separate, therefore they can be adjusted and optimized separately. These features provide detection for some organic gases in extremely low quantities and concentrations and in a manner which is compatible with many applications. Specifically, the potential applications for the MVID include analysis of planetary and cometary atmospheres, environmental monitoring, detection of drug, explosive and chemical agents, and as a novel ionization source for the above described ion mobility spectrometry processes.